This invention relates generally to composite membranes comprising a porous substrate and a polymeric composition comprising various combinations of xcex1,xcex2,xcex2-trifluorostyrene, substituted xcex1,xcex2,xcex2-trifluorostyrene and ethylene-based monomeric units. Where the polymeric composition includes ion-exchange moieties, the resultant composite membranes are useful in electrochemical applications, particularly as membrane electrolytes in electrochemical fuel cells.
Dense films can be obtained from solutions of poly-xcex1,xcex2,xcex2-trifluorostyrene. However, the brittleness of these films greatly limits their application. Films obtained from some sulfonated poly-xcex1,xcex2,xcex2-trifluorostyrenes can be used as ion-exchange membranes. However, such films often have unfavorable mechanical properties when wet, and are known to be very brittle in the dry state (see, for example, Russian Chemical Reviews, Vol. 59, p. 583 (1988)). Such films are of little practical use in fuel cells due to their poor physical properties. Some improvements in mechanical properties have been achieved by blending sulfonated poly-xcex1,xcex2,xcex2-trifluorostyrene with polyvinylidene fluoride and triethyl phosphate plasticizer, but these films remained unsatisfactory for application in electrochemical cells (see Fuel Cell Handbook, A. J. Appleby, published by Van Nostrand Reinhold, p. 286 (1989)).
U.S. Pat. No. 5,422,411 and the related patent applications mentioned above describe various polymeric compositions incorporating substituted xcex1,xcex2,xcex2-trifluorostyrenes and some cases further incorporating substituted ethylenes. Typically these compositions, as membranes, possess favorable mechanical properties compared to poly-xcex1,xcex2,xcex2-trifluorostyrene and sulfonated poly-xcex1,xcex2,xcex2-trifluorostyrene, although some of the membranes have a tendency to become brittle in the fully dehydrated state, depending, for example, on the equivalent weight. This effect is most apparent at equivalent weights below approximately 380 g/mol. Ion-exchange membranes derived from these polymeric compositions are suitable for many applications, including use in electrochemical applications, such as fuel cells.
For ease of handling, for example, in the preparation of membrane electrode assemblies for use in electrochemical fuel cells, the mechanical strength of the membrane in the dry state is important. In electrochemical applications, such as electrolytic cells and fuel cells, the dimensional stability (changes in the dimensions of the membrane due to changes in the degree of hydration) of the membrane during operation is also important. However, to improve performance, it is generally desirable to reduce membrane thickness and to decrease the equivalent weight (thereby increasing the water content) of the membrane electrolyte, both of which tend to decrease both the mechanical strength in the dry state and the dimensional stability in the wet state. One way to improve mechanical strength and dimensional stability in ionomeric membranes is through use of a substrate or support material, to give a composite membrane. The substrate is selected so that it imparts mechanical strength and dimensional stability to the membrane. The substrate material can be combined with the membrane polymeric material to form a composite membrane in a variety of ways. For example, if possible, an unsupported membrane can be preformed and then laminated to the porous substrate. Alternatively, a solution of the polymer can be impregnated into the porous substrate material, and the composite membrane subsequently dried. The formation of composite membranes via impregnation provides a more intimate contact between the two components, thus giving advantages over standard lamination approaches.
Composite ion-exchange membranes prepared by impregnating commercially available porous polytetrafluoroethylene film (Gore-tex(copyright)) with Nafion(copyright), a perfluorosulfonate ionomer, have been described in Journal of the Electrochemical Society, Vol. 132, pp. 514-515 (1985). The major goal in the study was to develop a composite membrane with the desirable chemical and mechanical features of Nafion(copyright), but which could be produced at low cost. Indeed, based on the polymer loadings necessary to produce these composite membranes, they are a low cost alternative to the costly perfluorosulfonic acid membranes. As indicated above, however, these perfluorosulfonate ionomers are known to form membranes suitable for use in electrochemical applications without the use of a substrate.
It has been discovered that polymers which have a tendency to become brittle in the dehydrated state can be rendered mechanically stable, even in the fully dehydrated state, by impregnation into suitable substrates.
Furthermore, it has been discovered that even polymers which are poor film formers, or polymers which form films with mechanical properties and dimensional stability which would preclude their use in electrochemical and other applications, can be made into composite membranes through incorporation into a suitable substrate. The resulting composite membranes have the desired physical properties for use in a wide range of applications.
In one aspect of the present invention, a composite membrane comprises a porous substrate impregnated with a polymeric composition comprising xcex1,xcex2,xcex2-trifluorostyrene monomeric units.
In another aspect, a composite membrane comprises a porous substrate impregnated with a polymeric composition comprising substituted xcex1,xcex2,xcex2-trifluorostyrene monomeric units. Substituted xcex1,xcex2,xcex2-trifluorostyrene monomeric units have at least one non-hydrogen substituent on the aromatic ring. In a preferred embodiment, the polymeric composition comprises at least two different substituted xcex1,xcex2,xcex2-trifluorostyrene monomeric units.
In a first embodiment of a composite membrane comprising a porous substrate impregnated with a polymeric composition comprising xcex1,xcex2,xcex2-trifluorostyrene monomeric units, the polymeric composition further comprises ethylene monomeric units, the polymeric composition derived from a copolymerization reaction involving at least ethylene and xcex1,xcex2,xcex2-trifluorostyrene.
In a second embodiment of a composite membrane comprising a porous substrate impregnated with a polymeric composition comprising xcex1,xcex2,xcex2-trifluorostyrene monomeric units, the polymeric composition further comprises partially fluorinated ethylene monomeric units, the polymeric composition derived from a copolymerization reaction involving at least xcex1,xcex2,xcex2-trifluorostyrene and, for example, CH2xe2x95x90CHF, CHFxe2x95x90CHF, CF2xe2x95x90CH2, or CF2xe2x95x90CHF.
In a third embodiment of a composite membrane comprising a porous substrate impregnated with a polymeric composition comprising xcex1,xcex2,xcex2-trifluorostyrene monomeric units, the polymeric composition further comprises tetrafluoroethylene monomeric units, the polymeric composition derived from a copolymerization reaction involving at least tetrafluoroethylene and xcex1,xcex2,xcex2-trifluorostyrene.
In a fourth embodiment of a composite membrane comprising a porous substrate impregnated with a polymeric composition comprising xcex1,xcex2,xcex2-trifluorostyrene monomeric units, the polymeric composition further comprises: 
where m is an integer greater than zero; Y is selected from the group consisting of chlorine, bromine, iodine, CxHyFz (where x is an integer greater than zero and y+z=2x+1), Oxe2x80x94R (where R is selected from the group consisting of CxHyFz (where x is an integer greater than zero and y+z=2x+1) and aryls), CFxe2x95x90CF2, CN, COOH and CO2R1 (where R1 is selected from the group consisting of perfluoroalkyls, aryls, and NR2R3 where R2 and R3 are selected from the group consisting of hydrogen, alkyls and aryls).
In a fifth embodiment of a composite membrane comprising a porous substrate impregnated with a polymeric composition comprising xcex1,xcex2,xcex2-trifluorostyrene monomeric units, the polymeric composition further comprises styrene monomeric units, the polymeric composition derived from a copolymerization reaction involving at least styrene and xcex1,xcex2,xcex2-trifluorostyrene.
In a sixth embodiment of a composite membrane comprising a porous substrate impregnated with a polymeric composition comprising xcex1,xcex2,xcex2-trifluorostyrene monomeric units, the polymeric composition further comprises substituted styrene monomeric units, the polymeric composition derived from a copolymerization reaction involving at least a substituted styrene and xcex1,xcex2,xcex2-trifluorostyrene. Substituted styrene monomeric units have at least one non-hydrogen substituent on the aromatic ring.
In a first embodiment of a composite membrane comprising a porous substrate impregnated with a polymeric composition comprising substituted xcex1,xcex2,xcex2-trifluorostyrene monomeric units, the polymeric composition comprises: 
where m is an integer greater than zero. In a further embodiment the polymeric composition comprises: 
where m is an integer greater than zero, and at least one of n, p and q is an integer greater than zero; A1, A2 and A3 are selected from the group consisting of hydrogen, halogens, CxHyFz (where x is an integer greater than zero and y+z=2x+1), CFxe2x95x90CF2, CN, NO2 and OH, Oxe2x80x94R (where R is selected from the group consisting of alkyls and perfluoroalkyls and aryls). In a still further embodiment, the group from which A1, A2 and A3 are selected further consists of SO3H, PO2H2, PO3H2, CH2PO3H2, COOH, OSO3H, OPO2H2, OPO3H2, NR3+ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls) and CH2NR3+ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls), and at least one of A1, A2 and A3 is selected from the group consisting of SO3H, PO2 H2, PO3H2, CH2PO3H2, COOH, OSC3H, OPO2H2, OPO3H2, NR3+ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls) and CH2NR3+ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls).
In a second embodiment of a composite membrane comprising a porous substrate impregnated with a polymeric composition comprising substituted xcex1,xcex2,xcex2-trifluorostyrene monomeric units, the polymeric composition comprises: 
where at least one of n, p and q is an integer greater than zero; A1, A2 and A3 are selected from the group consisting of CFxe2x95x90CF2, CN, NO2 and OH, Oxe2x80x94R (where R is selected from the group consisting of CxHyFz (where x is an integer greater than three and y+z=2x+1) and aryls).
In a third embodiment of a composite membrane comprising a porous substrate impregnated with a polymeric composition comprising substituted xcex1,xcex2,xcex2-trifluorostyrene monomeric units, the polymeric composition comprises: 
where m is an integer greater than zero.
In a fourth embodiment of a composite membrane comprising a porous substrate impregnated with a polymeric composition comprising substituted xcex1,xcex2,xcex2-trifluorostyrene monomeric units, the polymeric composition comprises: 
where m is an integer greater than zero; X is selected from the group consisting of PO2H2, PO3H2, CH2PO3H2, COOH, OSO3H, OPO2H2, OPO3H2, OArSO3H where Ar is an aryl, NR3, (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls) and CH2NR3+ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls).
In a fifth embodiment of a composite membrane comprising a porous substrate impregnated with a polymeric composition comprising substituted xcex1,xcex2,xcex2-trifluorostyrene monomeric units, the polymeric composition comprises: 
where m is an integer greater than zero, and at least one of n, p and q is an integer greater than zero; X is selected from the group consisting of SO3H, PO2H2, PO3H2, CH2PO3H2, COOH, OSO3H, OPO2H2, OPO3H2, CArSO3H where Ar is an aryl, NR3+ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls) and CH2NR3+ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls); A1, A2 and A3 are selected from the group consisting of halogens, CxHyFz (where x is an integer greater than zero and y+z=2x+1), CFxe2x95x90CF2, CN, NO2 and OH, Oxe2x80x94R (where R is selected from the group consisting of alkyls and perfluoroalkyls and aryls). In a further embodiment, the group from which A1, A2 and A3 are selected further consists of hydrogen. In a still further embodiment, the group from which A1, A2 and A3 are selected further consists of SO3H, PO2H2, PO3H2, CH2PO3H2, COOH, OSO3H, OPO2H2, OPO3H2, N3+ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls) and CH2N3+ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls), and at least one of A1, A2 and A3 is selected from the group consisting of SO3H, PO2H2, PO3H2, CH2PO3H2, COOH, OSO3H, OPO2H2, OPO3H2, NR3+ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls) and CH2NR3+ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls).
In a sixth embodiment of a composite membrane comprising a porous substrate impregnated with a polymeric composition comprising substituted xcex1,xcex2,xcex2-trifluorostyrene monomeric units, the polymeric composition comprises: 
where m is an integer greater than zero; B and D are selected from the group consisting of hydrogen, SO2F, SO3H, PO2H2, PO3H2, CH2PO3H2, COOH, OSO3H, OPO2H2, OPO3H2, NR3+ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls) and CH2NR3+ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls) In a further embodiment, the polymeric composition comprises: 
where m is an integer greater than zero, and at least one of n, p and q is an integer greater than zero; B and D are selected from the group consisting of hydrogen, SO2F, SO3H, PO2H2, PO3H2, CH2PO3H2, COOH, OSO3H, OPO2H2, OPO3H2, NR3+ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls) and CH2NR3+ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls); A1, A2 and A3 are selected from the group consisting of hydrogen, SO2F, halogens, CxHyFz (where x is an integer greater than zero and y+z=2x+1), CFxe2x95x90CF2, CN, NO2 and OH, Oxe2x80x94R (where R is selected from the group consisting of alkyls and perfluoroalkyls and aryls). In a still further embodiment, the group from which A1, A2 and A3 are selected further consists of SO3H, PO2H2, PO3H2, CH2PO3H2, COOH, OSO3H, OPO2H2, OPO3H2, NR3+ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls) and CH2NR3+ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls), and at least one of A1, A2 and A3 is selected from the group consisting of SO3H, PO2H2, PO3H2, CH2PO3H2, COOH, OSO3H, OPO2H2, OPO3H2, NR3+ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls) and CH2NR3+ (where R is selected from the group consisting of alkyls, perfluoroalkyls and aryls). In preferred embodiments B is hydrogen.
In a seventh embodiment of a composite membrane comprising a porous substrate impregnated with a polymeric composition comprising substituted xcex1,xcex2,xcex2-trifluorostyrene monomeric units, the polymeric composition further comprises ethylene monomeric units.
In an eighth embodiment of a composite membrane comprising a porous substrate impregnated with a polymeric composition comprising substituted xcex1,xcex2,xcex2-trifluorostyrene monomeric units, the polymeric composition further comprises partially fluorinated ethylene monomeric units, the polymeric composition derived from a copolymerization reaction involving, for example, CH2xe2x95x90CHF, CHFxe2x95x90CHF, CF2xe2x95x90CH2, or CF2xe2x95x90CHF.
In a ninth embodiment of a composite membrane comprising a porous substrate impregnated with a polymeric composition comprising substituted xcex1,xcex2,xcex2-trifluorostyrene monomeric units, the polymeric composition further comprises tetrafluoroethylene monomeric units.
In a tenth embodiment of a composite membrane comprising a porous substrate impregnated with a polymeric composition comprising substituted xcex1,xcex2,xcex2-trifluorostyrene monomeric units, the polymeric composition further comprises:
where m is an integer greater than zero; Y is selected from the group consisting of chlorine, bromine, iodine, CxHyFz (where x is an integer greater than zero and y+z=2x+1), Oxe2x80x94R (where R is selected from the group consisting of CxHyFz (where x is an integer greater than zero and y+z=2x+1) and aryls), CFxe2x95x90CF2, CN; COOH and CO2R1 (where R1 is selected from the group consisting of alkyls, perfluoroalkyls, aryls, and NR2R3 where R2 and R3 are selected from the group consisting of hydrogen, alkyls and aryls).
In an eleventh embodiment of a composite membrane comprising a porous substrate impregnated with a polymeric composition comprising substituted xcex1,xcex2,xcex2-trifluorostyrene monomeric units, the polymeric composition further comprises styrene monomeric units.
In a twelfth embodiment of a composite membrane comprising a porous substrate impregnated with a polymeric composition comprising substituted xcex1,xcex2,xcex2-trifluorostyrene monomeric units, the polymeric composition further comprises substituted styrene monomeric units. Substituted styrene monomeric units have at least one non-hydrogen substituent on the aromatic ring.
In the aspects and embodiments described above, the substrate is preferably a porous film or sheet material. For electrochemical applications, for example, preferred porous substrates comprise, or consist essentially of, porous polyolefins. Preferred polyolefins are polyethylene and polypropylene. Particularly preferred substrates comprise, or consist essentially of, porous polytetrafluoroethylene, also known as expanded polytetrafluoroethylene.
In a preferred aspect, a composite membrane comprises a porous substrate impregnated with a polymeric composition comprising: 
where m and n are integers greater than zero and A1 is selected from the group consisting of fluorine, CF3 and para-phenoxy. In a further embodiment of this preferred aspect, the group from which A1 is selected further consists of hydrogen.
In another preferred aspect, a composite membrane comprises a porous substrate impregnated with a polymeric composition comprising: 
where m, n, and p are integers greater than zero and A1 and A2 are selected from the group consisting of hydrogen, fluorine, CF3, and paraphenoxy.
In another preferred aspect, a composite membrane comprises a porous substrate impregnated with a polymeric composition comprising: 
where m and n are integers greater than zero and X is selected from the group consisting of para-SO2F, meta-SO3H and para-SO3H.
In yet another preferred aspect, a composite membrane comprises a porous substrate impregnated with a polymeric composition comprising: 
where m and q are integers greater than zero, n and p are zero or an integer greater than zero; X is selected from the group consisting of para-SO2F, meta-SO3H and para-SO3H; and A1 and A2 are selected from the group consisting of hydrogen, fluorine, CF3, and para-phenoxy. In a further embodiment of this preferred aspect, n is an integer greater than zero.
In still another preferred aspect, a composite membrane comprises a porous substrate impregnated with a polymeric composition comprising: 
where m and q are integers greater than zero, n and p are zero or an integer greater than zero; X is selected from the group consisting of para-SO2F, meta-SO3H and para-SO3H; and A1 and A2 are selected from the group consisting of hydrogen, fluorine, CF3, and para-phenoxy. In a further embodiment of this preferred aspect, n is an integer greater than zero.
In the aspects and embodiments described above, the polymeric compositions can consist essentially of the described monomeric units.
In all of the above preferred aspects, preferably the porous substrate comprises polytetrafluoroethylene. A preferred porous substrate consists essentially of polytetrafluoroethylene.
In the aspects and embodiments described above, the A1, A2, A3 substituents may be further elaborated by known means such as, for example, by hydrolysis of the CN group to form COOH or by reduction with common reducing agents (such as, for example, Raney nickel) to form a primary amine, thereby transforming the A1, A2 and A3 substituents into ion-exchange moieties. The resulting olymeric composition may thus comprise one or more type of ion-exchange moiety, and may also comprise both cation-exchange and anion-exchange moieties.
The term xe2x80x9cmonomeric unitxe2x80x9d as used herein indicates that the polymeric composition contains the described fragment or unit, and is obtained by a polymerization reaction involving the corresponding unsaturated monomer.
The substituents on the aromatic rings (including, for example, A1, A2, A3, X, B and D) may each be located in the ortho, meta or para positions, as indicated in the formulas wherein the chemical bond drawn for the substituents intersects the aromatic ring. In preferred aspects of the described embodiments, the substituents are in the meta or para positions.
As used herein, the term xe2x80x9carylxe2x80x9d refers to a substituted or unsubstituted phenyl group. The formula CxHyFz (where x is an integer greater than zero and y+z=2x+1) is used to indicate alkyl, perfluoroalkyl or partially fluorinated alkyl groups.
In accordance with convention in the art, the above chemical formulas for polymeric compositions containing more than two monomeric units (where at least three of m, n, p and q are greater than zero) are intended to indicate that the monomeric units are present in the polymeric composition, but are not limited to the particular order in which the monomeric units are set forth in each general formula. For example, random linear copolymers, alternating copolymers and linear block copolymers, formed from the indicated monomeric units, are contemplated.